1. Field of the Invention
The present invention relates to a semiconductor device which is provided with a circuit including a semiconductor element such as a transistor. For example, the present invention relates to a power device which is mounted on a power supply circuit, a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like, an electro-optical device typified by a liquid crystal display device, a light-emitting display device including a light-emitting element, and the like; and an electronic device on which any of the above circuit and devices are mounted as a component.
In this specification, a semiconductor device generally refers to a device which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors formed using amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors formed using polycrystalline silicon have high field-effect mobility, they are not suitable for being formed over a larger glass substrate.
Other than a transistor formed using silicon, in recent years, a technique in which a transistor is formed using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, a technique of forming a transistor by using zinc oxide or In—Ga—Zn—O-based oxide as an oxide semiconductor, and of using the transistor for a switching element of a pixel of a display device and the like is disclosed in Patent Document 1 and Patent Document 2.
In an oxide semiconductor, part of hydrogen serves as a donor to release electrons as carriers. When the carrier concentration in the oxide semiconductor becomes high, a channel is formed in a transistor without voltage application to a gate. That is, the threshold voltage of the transistor shifts to the negative direction. It is difficult to remove hydrogen in the oxide semiconductor completely; therefore, it is also difficult to control the threshold voltage.
In Patent Document 3, it is disclosed that when hydrogen is added in an oxide semiconductor film, electrical conductivity of the oxide semiconductor is increased by approximately four to five orders of magnitude. In addition, it is disclosed that hydrogen is diffused into the oxide semiconductor film from an insulating film which is in contact with the oxide semiconductor film.